STP Engine Flushis a specially designed formula that helps
neutralise engine acids as well as removing sludge, varnish and gum
deposits that normal draining leaves behind. Simply add contents to
engine oil and run the engine. Drain while still hot and replace filter
and engine oil.

Technical information:

STP Engine Flush is specially formulated to rapidly dissolve harmful
engine deposits and maintain efficient oil circulation throughout the
entire engine, helping to protect vital engine wear surfaces throughout
the flush. Even the finest of engine oil will leave acid deposits and
other sludge. This causes your engine oil to be ineffective.